U.S. patent number 10,792,909 [Application Number 15/571,099] was granted by the patent office on 2020-10-06 for laser-engravable pad printing plate.
This patent grant is currently assigned to Flint Group Germany GmbH. The grantee listed for this patent is Flint Group Germany GmbH. Invention is credited to Alfred Leinenbach, Markus Muhlfeit, Jochen Unglaube.
United States Patent |
10,792,909 |
Muhlfeit , et al. |
October 6, 2020 |
Laser-engravable pad printing plate
Abstract
A laser-engravable pad printing plate comprising at least (a) a
metal support, (b) an adhesion layer, (c) a laser-engravable
recording layer having a layer thickness of 20 .mu.m to 200 .mu.m,
(d) a cover film, characterized in that the laser-engravable
recording layer (c) comprises (c1) 40 to 95 wt % of a polyvinyl
alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to 30 wt % of
an inorganic filler, (c4) 0 to 20 wt % of a crosslinker, and (c5) 0
to 10 wt % of further additives.
Inventors: |
Muhlfeit; Markus (Weil der
Stadt, DE), Leinenbach; Alfred (Oberkirch-Nu bach,
DE), Unglaube; Jochen (Kenzingen, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Flint Group Germany GmbH |
Stuttgart |
N/A |
DE |
|
|
Assignee: |
Flint Group Germany GmbH
(DE)
|
Family
ID: |
1000005095145 |
Appl.
No.: |
15/571,099 |
Filed: |
May 3, 2016 |
PCT
Filed: |
May 03, 2016 |
PCT No.: |
PCT/EP2016/059844 |
371(c)(1),(2),(4) Date: |
November 01, 2017 |
PCT
Pub. No.: |
WO2016/177702 |
PCT
Pub. Date: |
November 10, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180186146 A1 |
Jul 5, 2018 |
|
Foreign Application Priority Data
|
|
|
|
|
May 4, 2015 [EP] |
|
|
15166176 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B32B
27/36 (20130101); B32B 37/182 (20130101); B41F
17/001 (20130101); B32B 15/09 (20130101); B41N
1/06 (20130101); B32B 15/18 (20130101); B41N
1/20 (20130101); B41M 1/40 (20130101); B41M
1/10 (20130101); B41C 1/05 (20130101); B23K
26/364 (20151001); B32B 2255/06 (20130101); B32B
2255/10 (20130101); B41C 2201/04 (20130101); B32B
2255/26 (20130101); B41C 2201/02 (20130101); B41C
2201/14 (20130101); B32B 2311/30 (20130101); B32B
2250/02 (20130101); B32B 2367/00 (20130101) |
Current International
Class: |
B41C
1/05 (20060101); B32B 27/36 (20060101); B41F
17/00 (20060101); B32B 37/18 (20060101); B41N
1/06 (20060101); B32B 15/09 (20060101); B41M
1/10 (20060101); B32B 15/18 (20060101); B23K
26/364 (20140101); B41N 1/20 (20060101); B41M
1/40 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
1172227 |
|
Jan 2002 |
|
EP |
|
1177911 |
|
Feb 2002 |
|
EP |
|
1701852 |
|
Sep 2006 |
|
EP |
|
2047987 |
|
Apr 2009 |
|
EP |
|
2767408 |
|
Aug 2014 |
|
EP |
|
WO-2006061053 |
|
Jun 2006 |
|
WO |
|
Other References
English Translation of International Preliminary Report for
Patentability for International Application No. PCT/EP2016/059844,
dated Apr. 12, 2018. cited by applicant .
International Preliminary Report on Patentability for
PCT/EP2016/059844 dated Apr. 18, 2017 (in German). cited by
applicant .
International Search Report for PCT/EP2016/059844 dated Jun. 14,
2016. cited by applicant .
Written Opinion of the International Searching Authority for
PCT/EP2016/059844 dated Jun. 14, 2016. cited by applicant.
|
Primary Examiner: Zimmerman; Joshua D
Attorney, Agent or Firm: Faegre Drinker Biddle & Reath
LLP
Claims
The invention claimed is:
1. A laser-engravable pad printing plate comprising: (a) a metal
support, (b) an adhesion layer, (c) a laser-engravable recording
layer having a layer thickness of 20 .mu.m to 200 .mu.m obtained
from a mixture comprising (c1) 40 to 95 wt % of a polyvinyl
alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to 30 wt % of
an inorganic filler, (c4) 0.1 to 20 wt % of a crosslinker, and (c5)
0 to 10 wt % of further additives, (d) a cover film, wherein the
crosslinker (c4) is selected from the group consisting of
polyfunctional isocyanates, mono- or polyfunctional aldehydes,
polyfunctional epoxides, polyfunctional carboxylic acids and
polyfunctional carboxylic anhydrides; and wherein the
laser-engravable recording layer is obtained in a process
comprising chemically crosslinking the crosslinker with the
polyvinyl alcohol.
2. The laser-engravable pad printing plate as claimed in claim 1,
wherein the laser-engravable recording layer (c) comprises as
polyvinyl alcohol (c1) a partially hydrolyzed polyvinyl alcohol
ester having a degree of hydrolysis of 50 to 98 mol %.
3. The laser-engravable pad printing plate as claimed in claim 1,
wherein the recording layer (c) comprises as IR absorber (c2)
carbon black, graphite or carbon nanoparticles.
4. The laser-engravable pad printing plate as claimed in claim 1,
wherein the recording layer (c) comprises 5 to 30 wt % of an
inorganic filler.
5. The laser-engravable pad printing plate as claimed in claim 1,
wherein the recording layer (c) comprises an inorganic filler (c3)
having a hardness of >4 Mohs.
6. The laser-engravable pad printing plate as claimed in claim 1,
wherein the recording layer (c) comprises as inorganic filler (c3)
a finely ground quartz whose surface has been modified with
silanes.
7. The laser-engravable pad printing plate as claimed in claim 1,
wherein the sum total of IR absorber (c2) and inorganic filler (c3)
in the recording layer is <50 wt %.
8. The laser-engravable pad printing plate as claimed in claim 7,
wherein the crosslinker is glyoxal or glutaraldehyde.
9. The laser-engravable pad printing plate as claimed in claim 1,
wherein the adhesion layer is a 2-component polyurethane adhesion
varnish.
10. The laser-engravable pad printing plate as claimed in claim 1,
wherein the metal support (a) is a steel plate having a thickness
of 50 to 300 .mu.m.
11. The laser-engravable pad printing plate as claimed in claim 1,
wherein the cover film is a PET film having a mean roughness depth
Rz of 0.3 to 3 .mu.m.
12. A method for producing a pad printing plate comprising: (a) a
metal support, (b) an adhesion layer, (c) a laser-engravable
recording layer having a layer thickness of 20 .mu.m to 200 .mu.m,
(d) a PET cover film, wherein the laser-engravable recording layer
(c) is made from a mixture which comprises (c1) 40 to 95 wt % of a
polyvinyl alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to
30 wt % of an inorganic filler, (c4) 0.1 to 20 wt % of a
crosslinker, and (c5) 0 to 10 wt % of further additives, wherein
the laser-engravable recording layer is obtained by a process
comprising chemically crosslinking the crosslinker with the
polyvinyl alcohol; the method comprising steps (i) to (iii): (i)
coating the metal support with the adhesion layer, (ii) applying
the laser-engravable recording layer to the PET cover film and
drying the recording layer in one or more steps, (iii) laminating
the coated PET cover film onto the metal support coated with the
adhesion layer; wherein the crosslinker (c4) is selected from the
group consisting of polyfunctional isocyanates, mono- or
polyfunctional aldehydes, polyfunctional epoxides, polyfunctional
carboxylic acids and polyfunctional carboxylic anhydrides.
13. A method for producing a pad printing cliche from a
laser-engravable pad printing plate as defined in claim 12, further
comprising steps (iv) to (vi): (iv) removing the PET cover film
from the pad printing plate, (v) engraving the depressions into the
laser-engravable recording layer by means of an IR laser, (vi)
cleaning the laser-engraved pad printing cliche by rinsing with a
solvent.
14. A method for printing a substrate by the pad printing process
with a pad printing cliche obtainable by the method of claim 13,
further comprising steps (vii) to (x): (vii) fastening the pad
printing cliche in the pad printing machine, (viii) inking the pad
printing cliche with a solvent-based pad printing ink, (ix)
stripping off the excess printing ink by means of a doctor blade,
(x) transferring the printing ink by means of a rubber pad onto the
substrate to be printed.
15. A laser-engravable pad printing plate comprising: (e) a metal
support, (f) an adhesion layer, (g) a laser-engravable recording
layer having a layer thickness of 20 .mu.m to 200 .mu.m obtained
from a mixture comprising (c6) 40 to 95 wt % of a polyvinyl
alcohol, (c7) 5 to 50 wt % of an IR absorber, (c8) 0 to 30 wt % of
an inorganic filler, (c9) 0.1 to 20 wt % of a crosslinker, and
(c10) 0 to 10 wt % of further additives, (h) a cover film, wherein
the crosslinker (c4) is selected from the group consisting of mono-
or polyfunctional aldehydes, polyfunctional epoxides, and
polyfunctional carboxylic anhydrides; wherein the laser-engravable
recording layer is obtained by a process comprising chemically
crosslinking the crosslinker with the polyvinyl alcohol.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a national stage application (under 35 U.S.C.
.sctn. 371) of PCT/EP2016/059844, filed May 3, 2016, which claims
benefit of European Application No. 15166176.6, filed May 4, 2015,
both of which are incorporated herein by reference in their
entirety.
Pad printing is an indirect gravure process which is used for
printing a wide variety of different materials and shapes,
including three-dimensional objects. In this printing process, a
printing cliche with depressions (wells, letters, line elements) is
inked with printing ink. The excess printing ink is removed by
doctor blade from the surface of the printing forme. At this point
a soft rubber element, known as the pad, is pressed onto the
surface of the printing forme, and the printing ink is transferred
from the depressions in the printing cliche to the pad. The pad is
subsequently lifted up and moved horizontally and then lowered
again onto the object that is to be printed. In order to achieve
ink transfer that is as complete as possible, it is usual for
silicone-based rubber materials to be used.
The doctor blade operation imposes exacting requirements on the
mechanical stability of the printing cliche. Any damage to the
cliche surface--scratches resulting from dust particles,
contaminations of the printing ink, or an uneven doctor
blade--results in tinting of the nonimage areas and renders the
cliche unusable. For long runs, therefore, steel cliches are used,
into which the depressions are engraved by means of photographic
mask techniques and etching techniques, at high cost and
complexity.
Alternative printing cliches are based on photopolymeric layer
materials. These materials comprise a photopolymerizable layer on a
stable metal support. The photopolymerizable layer is exposed by
means of UVA light through a film. The exposed areas of the
recording layer are crosslinked and become insoluble; the unexposed
areas of the recording layer remain soluble and are removed in a
subsequent washing step. Binders mentioned are polyvinyl alcohol
and polyamide. To increase the scratch resistance, mineral fillers
are added to the photopolymeric layer. Furthermore, as
crosslinkers, descriptions are given of specific bifunctional
acrylates, which afford hard, scratch-resistance materials after
exposure to UVA light. The process, however, is a costly and
inconvenient one, and the photographic films are expensive.
EP 767 408 therefore describes photopolymerizable materials which,
on the photopolymeric layer, have a thin, black mask layer into
which the image information can be written by means of laser. The
photopolymerizable layer is subsequently exposed with UVA light
through the mask produced, and is crosslinked. The unexposed areas
are again removed in a washing step. In this process there is no
longer any need for a film. Nevertheless, the process remains
costly and inconvenient, and the depth of the individual wells is
difficult to control.
It would be simpler to engrave the depressions into the printing
forme directly by means of a laser. For example, single-layer or
multilayer metal printing formes are known into which the
depressions are engraved directly by means of a laser. In that
case, a disruptive burr is formed around the engraved image
elements, arising as a result of the melting of the metal. This
burr has to be removed, at cost and inconvenience, in order to
obtain a satisfactory doctor blade outcome.
Ceramic cliches may likewise be engraved directly using suitable
lasers, as described in EP 1 701 852, for example. Ceramic
materials have less of a tendency to melt and so do not require
afterwork. Nevertheless, producing the ceramic layers in the
accuracy of thickness required is expensive and inconvenient.
EP 1 172 227 therefore proposes the application, to a metal
support, of a one-component or two-component coating material in a
layer thickness of 0.1 to 0.3 mm, and the engraving of the relief
into the cured coating material. The chemical composition of the
coating material, however, is not described, and so it is unknown
how sufficient doctor blade resistance and a satisfactory,
burr-free engraving outcome can be achieved with such a setup.
It is an object of the invention to provide a laser-engravable
printing plate for pad printing that can be manufactured
inexpensively, does not require costly and inconvenient afterwork
after laser engraving, and exhibits the required doctor blade
resistance.
The object is achieved by means of a laser-engravable pad printing
plate comprising at least (a) a metal support, (b) an adhesion
layer, (c) a laser-engravable recording layer having a layer
thickness of 20 .mu.m to 200 .mu.m, (d) a cover film, characterized
in that the laser-engravable recording layer (c) comprises (c1) 40
to 95 wt % of a polyvinyl alcohol, (c2) 5 to 50 wt % of an IR
absorber, (c3) 0 to 30 wt % of an inorganic filler, (c4) 0 to 20 wt
% of a crosslinker, and (c5) 0 to 10 wt % of further additives.
The thickness of the laser-engravable recording layer (c) is guided
by the required depth of the image elements to be engraved. In pad
printing, these elements are generally a few .mu.m up to a maximum
of 50 .mu.m deep. For certain specialty applications, where high
quantities of ink have to be transferred, image element depths of
up to 150 .mu.m are required. Generally speaking, however, relief
depths of up to 30 .mu.m are sufficient. The thickness of the
laser-engravable recording layer is therefore in a range from 20 to
200 .mu.m, preferably from 20 to 50 .mu.m, more particularly from
20 to <50 .mu.m. The figures are based on the dried recording
layer.
The pad printing cliches are printed predominantly using printing
inks which are apolar or of moderate polarity, and so relatively
polar binders are appropriate for reasons of swelling resistance
relative to organic solvents. The binder ought additionally to have
good dispersing properties for the reinforcing inorganic fillers,
to have good mechanical properties, to be laser-engravable as far
as possible without melt edges, and to give the laser-engraved
recording layer (printing cliche) very good doctor blade
resistance.
Surprisingly it has been found that laser-engravable lasers for pad
printing plates with outstanding doctor blade resistance can be
obtained using polyvinyl alcohols as binders.
Furthermore, the laser-engravable layers of the invention, based on
polyvinyl alcohol, have very good swelling resistance with respect
to organic solvents, and so the resulting printing cliches can be
printed effectively using apolar or moderately polar printing inks.
Moreover, the polyvinyl alcohol has good dispersing properties for
reinforcing inorganic fillers and produces recording layers which
are laser-engravable without melt edges and exhibit good mechanical
properties.
Polyvinyl alcohols are polymers with vinyl alcohol units, more
particularly partially or fully hydrolyzed polyvinyl acetates.
Vinyl acetate-vinyl alcohol copolymers are characterized by the
molecular weight and the degree of hydrolysis (the percentage
fraction of the vinyl alcohol units in the polymer, based on the
total number of monomer units). The crystallinity of the products
and their mechanical properties can be controlled according to the
degree of hydrolysis. The polar nature of the OH groups is
responsible, furthermore, for the good dispersing capacity of the
polyvinyl alcohols. Polyvinyl alcohols are highly suitable from the
standpoint of laser engravability as well. They do not exhibit
pronounced melting, instead decomposing when exposed to high
temperatures--of the kind which arise in the course of laser
engraving--without residue. The recording layers formulated with
polyvinyl alcohols as binders can be engraved without burring by
means of IR lasers.
For the printing plates of the invention, partially hydrolyzed
polyvinyl alcohol esters with a moderate to high degree of
hydrolysis are preferred. If the degree of hydrolysis is too low,
the polymers are too soft and are no longer resistant toward apolar
pad printing inks. At very high degrees of hydrolysis, the products
become too hard and brittle, with adverse consequences in
particular for the doctor blade resistance/scratch resistance of
the pad printing cliches. In general, polyvinyl alcohols (c1) used
are polyvinyl alcohols having a degree of hydrolysis of 50% to 98%.
Preferred for use are polyvinyl alcohols having a degree of
hydrolysis of 60% to 90%.
Instead of or together with vinyl acetate-vinyl alcohol copolymers
it is also possible for other vinyl alcohol copolymers, such as,
for example, polyvinyl propionate-vinyl alcohol), or
poly(ethylene-vinyl alcohol), to be used as binders (c1) in the
laser-engravable recording layer (c), provided they contain at
least 50 mol % of vinyl alcohol units.
Preference is therefore given to a laser-engravable recording layer
(c) which as polyvinyl alcohol (c1) comprises a partially
hydrolyzed polyvinyl alcohol ester having a degree of hydrolysis of
50 to 98 mol %.
Suitable IR absorbers for the recording layer include, in
particular, finely divided carbon black, graphite, or carbon black
nanoparticles. Carbon black has a broad absorption spectrum, which
extends from the visible range into the IR range. Layers containing
carbon black can therefore be engraved with all commercially
customary lasers such as, for example, IR laser diodes (830 nm) or
Nd:YAG solid-state or fiber lasers (1064 nm) or CO.sub.2 lasers
(10.6 .mu.m). Of course, the laser-engravable recording layer may
also comprise other IR absorbers, based on pigments, or soluble
dyes. Dyes which may be used include, for example, phthalocyanines
and substituted phthalocyanine derivatives, cyanine and merocyanine
dyes, or else polymethine dyes or azo dyes. These dyes absorb in
the near IR region, and so can be engraved using laser diodes (830
nm) and Nd:YAG lasers (1064 nm).
The recording layer (c) preferably comprises as IR absorber (c2)
carbon black, graphite, or carbon nanoparticles.
Particularly preferred for use as IR absorber is carbon black,
since the carbon black acts simultaneously as a mechanical
reinforcing agent and increases the mechanical resistance of the
laser-engravable recording layer.
The amount of the materials (c2) absorbing IR light is 5 to 50 wt
%, relative to the amount of all components of the laser-engravable
recording layer. An amount of 10 to 30 wt % is preferred.
A further component of the laser-engravable recording layer
comprises mineral fillers, which mechanically reinforce the layer
and so endow the layer with the necessary scratch resistance and
doctor blade resistance.
Suitable fillers include, in particular, hard, inorganic fillers
and pigments. Examples of particularly suitable fillers are silicon
dioxide, especially finely ground quartzes and quartz powders,
silicates, especially aluminum silicates, silicate glasses,
aluminum oxides, especially corundum, titanium dioxide, silicon
carbide, tungsten carbide, and similar hard minerals. Examples of
suitable pigments are iron oxides or chromium oxides.
The hardness of the fillers ought to be >4.0 on the Mohs
hardness scale. The average particle diameter of the inorganic
fillers is generally 0.1 .mu.m to 6 .mu.m. Less than 5% of the
particles, preferably less than 1% of the particles, ought to be
larger than 10 .mu.m. The shape of the fillers is arbitrary. The
majority of hard fillers do not form round particles, but instead
have arbitrary crystalline forms. The length of the individual
crystals (measured under the microscope), however, ought preferably
not to be more than 10 .mu.m.
The fillers may be surface-treated or coated, so as to be
particularly effectively dispersible in the polymeric matrix.
Surface-treated finely ground quartzes are preferred, since they
have the necessary hardness and permit effective attachment to the
polymeric matrix. Particularly preferred are finely ground quartzes
whose surfaces have been pretreated using silanes (aminosilane,
epoxy silane, methacryloylsilane, methylsilane, and vinylsilane)
and which can simply be dispersed uniformly into the polyvinyl
alcohol solution by means of stirred incorporation.
The mineral filler is present preferably in the recording layer,
generally in amounts of 5 to 30 wt %. The amount of IR absorber
(c2) and mineral filler (c3) in total is not more than 60 wt %.
Preferably it is not more than 50 wt %, based on all components of
the laser-engravable recording layer.
Preferably, therefore, the recording layer (c) comprises 5 to 30 wt
% of an inorganic filler (c3), more particularly of an inorganic
filler (c3) having a hardness of >4 Mohs. In one embodiment of
the invention, the recording layer (c) comprises, as inorganic
filler (c3), a finely ground quartz whose surface has been modified
with silanes.
In a further embodiment of the invention, the recording layer,
further to the mechanical reinforcement by the IR absorber (c2),
preferably carbon black, and optionally by the inorganic filler
(c3), is chemically crosslinked as well. The noncrosslinked
recording layers of polyvinyl alcohol, carbon black, and inorganic
filler do already have good doctor blade resistance. This
resistance, however, can be increased still further by chemical
crosslinking of the polyvinyl alcohol. The resistance of the
recording layer with respect to high atmospheric humidity is also
improved considerably if the polyvinyl alcohol is chemically
crosslinked. Noncrosslinked recording layers comprising polyvinyl
alcohol must be handled cautiously. In order to avoid fingerprints
on the surface of the printing plate, the wearing of gloves is
advisable. Recording layers based on crosslinked polyvinyl alcohol,
in contrast, are relatively insensitive toward moisture or
fingerprints, and so no particular measures are necessary during
the handling of the pad printing plates.
For the chemical crosslinking of the recording layer based on
polyvinyl alcohols, a variety of crosslinkers are contemplated,
such as polyfunctional isocyanates, mono- or polyfunctional
aldehydes, polyfunctional epoxides, polyfunctional carboxylic
acids, and polyfunctional carboxylic anhydrides.
Suitable polyfunctional isocyanates are toluene 2,4-diisocyanate
(TDI), methylenediphenyl diisocyanate (MDI), hexamethylene
diisocyanate (HDI, HMDI), polymeric methylenediphenyl diisocyanate
(PMDI), isophorone diisocyanate (IPDI),
4,4'-diisocyanatodicyclohexylmethane (H12MDI), and also blocked
aromatic polyisocyanates based on TDI and blocked aliphatic
polyisocyanates based on HDI,
Suitable monofunctional aldehydes are formaldehyde, acetaldehyde,
propionaldehyde, valeraldehyde, capronaldehyde, and pivalaldehyde.
Suitable polyfunctional aldehydes are glyoxal, glutaraldehyde
(1,5-pentanedial), succinaldehyde (butanedial), and
terephthalaldehyde.
Suitable polyfunctional epoxides are 1,2,3,4-diepoxybutane,
1,2,5,6-diepoxyhexane, 1,2,7,8-diepoxyoctane, and also epoxy resins
such as bisphenol A diglycidyl ether, or epoxyphenol novolaks.
Suitable polyfunctional carboxylic acids are oxalic acid, malonic
acid, succinic acid, glutaric acid, adipic acid, pimelinic acid,
suberic acid, azelaic acid, sebacic acid, tartaric acid, citric
acid, terephthalic acid, phthalic acid, asparatic acid, and
glutaminic acid.
Suitable polyfunctional carboxylic anhydrides are maleic anhydride,
succinic anhydride, and phthalic anhydride.
Particularly preferred among the crosslinking reactions is the
reaction of polyvinyl alcohols with polyfunctional isocyanates,
polyfunctional epoxides, or mono- or polyfunctional aldehydes.
These crosslinking reactions proceed virtually quantitatively at
temperatures which are not too high. Preferred crosslinkers are
aldehydes such as formaldehyde, acetaldehyde, propionaldehyde,
butyraldehyde, and higher aldehydes. In addition to their high
reactivity, a further advantage of these crosslinkers is that they
consist only of carbon, oxygen, and hydrogen. Accordingly, in
conjunction with polyvinyl alcohols, recording layers are produced
which on laser engraving release considerably fewer toxic gases
than recording layers which comprise nitrogen-containing
components, such as isocyanates, for example.
Particular preference is given to crosslinking with polyfunctional
aldehydes such as, for example, glyoxal, glutaraldehyde or
glyoxylic acid. Especially preferred is glyoxal. This aldehyde is
already in the form of an aqueous solution and can therefore be
admixed readily to aqueous or alcoholic polyvinyl alcohol
solutions. The solutions are sufficiently stable at room
temperature and can therefore be applied in layers effectively.
After the coating, the layers are briefly heated to temperatures of
100.degree. C. to 150.degree. C. Under these conditions, the
glyoxal reacts with the OH groups of the polyvinyl alcohol via the
hemiacetals to form the more stable acetals. Polyvinyl alcohol is
subsequently crosslinked. The recording layer can no longer be
finely dispersed or dissolved by water or a water/alcohol
mixture.
There is a further advantage to the crosslinking of the recording
layer. It may be necessary to achieve the desired dry film
thickness by building up the layer in a plurality of individual
coating operations. Where a noncrosslinked layer is coated over
with an identical coating solution, this layer is partially
dissolved. There are leveling defects which are detrimental to the
coating quality. A crosslinked layer, in contrast, can be readily
coated over. By building up the layer in a plurality of coating
operations, therefore, it is possible to achieve very much higher
layer thicknesses.
The crosslinker is present in the recording layer in general in
amounts of 0.1 to 20 wt %, based on the amount of all components of
the laser-engravable recording layer. The fraction of crosslinker
is preferably from 1 to 10 wt %.
Preferably, therefore, further to components (c1), (c2), and
optionally (c3), the laser-engravable recording layer (c)
additionally contains 0.1 to 20 wt % of a crosslinker (c4).
Preferred crosslinkers are glyoxal or glutaraldehyde.
Besides components (c1), (c2), optionally (c3), and optionally
(c4), the recording layer may comprise other components such as
plasticizers, flow control assistants, and dispersing assistants as
further additives (c5). Additives are generally present in the
recording layer in amounts of 0 to 10 wt %, preferably 0.1 to 5.0
wt %, based on the sum total of all components (c1) to (c5). The
sum total of the components (c1) to (c5) makes 100 wt %.
The laser-engravable recording layer is located on a metallic
support material. Aluminum or steel is preferred as metallic
support. Steel has the advantage that in that case the pad printing
cliche can be fixed magnetically in the pad printing machine. The
thickness of the metallic support may be 0.05 mm to 1 mm. If steel
is used as support material, preference is given to steel plates
having a thickness of 0.05 to 0.3 mm. Tin-plated steel plates are
used with preference for protection against corrosion.
For the effective attachment of the laser-engravable recording
layer to the metallic support (a), at least one adhesion layer (b)
is applied to said support. Examples of highly suitable adhesion
layers are 2-component polyurethane adhesion varnishes comprising a
polyol and a polyfunctional isocyanate. Alternatively, it is
possible to use epoxide varnishes or radically curing adhesion
varnishes. For more effective attachment of the recording layer to
the metal support, the adhesion varnishes may comprise pigments or
further additives. The thickness of the adhesion varnish layer or
of the various adhesion layers is generally a few micrometers,
preferably 1 .mu.m to 20 .mu.m.
To protect the printing plate surface, the pad printing plates of
the invention have a cover film which is not removed until before
the processing of the pad printing plate to the printing cliche.
Highly suitable are PET cover films which are 50 .mu.m to 200 .mu.m
thick and have an average roughness. PET films having an averaged
roughness depth of between 0.3 .mu.m and 3 .mu.m are preferred for
use as cover films.
The individual layers of the pad printing plate of the invention
can be applied in any desired way, by spraying, rolling or doctor
blade processes.
In one preferred embodiment, the metallic support is first of all
coated with the adhesion varnish or varnishes. In parallel with
this, the laser-engravable recording layer is applied to the PET
cover film. This is done by dissolving the polyvinyl alcohol in
water or water/alcohol mixtures, after which carbon black is added.
The suspension is then dispersed for a number of hours in a ball
mill, producing fine dispersal of the carbon black. The remaining
components of the recording layer are subsequently added. The
solution is then coated onto the PET film in one or more steps, and
thereafter dried.
In the course of drying, the recording layer is heated briefly to
temperatures between 100 and 150.degree. C., and the polyvinyl
alcohol is consumed by reaction with the crosslinker--if present in
the recording layer. The necessary drying times and/or reaction
times are in the region of a few seconds. The coated cover film is
subsequently laminated onto the metallic support. This laminating
process may be carried out dry, under the action of heat, or with
the aid of a laminating solvent.
If the recording layer is produced not in a single coating
operation, but instead in a plurality of consecutive partial
coating operations, it is advantageous if the recording layer
comprises a crosslinker. In that case the layer applied first
undergoes crosslinking on drying and can subsequently be readily
coated over with an identical coating solution. A noncrosslinked
layer, in contrast, cannot be coated over. As observed, there are
leveling defects, and the quality of coating is adversely
affected.
If the recording layer is first coated onto a cover film and only
then laminated onto the metallic support, better printing outcomes
are obtained, surprisingly, than if the individual layers are
applied successively to the metallic support and lined at the end
with a cover film. Presumably, when the recording layer is poured
on, the surface geometry of the PET film is imaged onto this layer,
and a more uniform surface is produced, thereby enhancing the
sliding behavior of the doctor blade in the pad printing
machine.
Preference is therefore given to a method for producing a pad
printing plate of the invention, comprising the steps of (i)
coating the metal support with the adhesion layer, (ii) applying
the laser-engravable recording layer to the PET cover film and
drying the recording layer, (iii) laminating the coated PET cover
film onto the metal support coated with the adhesion layer.
Another subject of the present invention is a method for producing
a pad printing cliche from a laser-engravable pad printing plate of
the invention, comprising steps (iv) to (vi): (iv) removing the
cover film from the pad printing plate, (v) engraving the
depressions into the laser-engravable recording layer by means of
an IR laser, (vi) cleaning the laser-engraved pad printing cliche
by rinsing with a solvent.
After the removal of the cover film, the depressions are engraved
with the aid of a laser into the recording layer of the pad
printing plate. If carbon black is used as IR absorber, the pad
printing plates of the invention can be engraved with all
commercially customary lasers, such as, for example, IR laser
diodes (830 nm) or Nd:YAG solid-state or fiber lasers (1064 nm), or
CO.sub.2 lasers (10.6 .mu.m). With regard to fineness of the laser
engraving, or maximum resolution of engraving, the basic rule is
that Nd:YAG lasers, or laser diodes which operate in the near IR,
respectively, are superior to the CO.sub.2 lasers.
In the case of very fine lines or image elements, they are written
into the recording layer without further processing of the data. In
the case of high-quality applications, or when relatively large
image elements are to be printed, it is usual to provide additional
screening of the image elements, in order to give the doctor blade
a resting surface and to prevent sinking of the blade into the
image elements. In order to enable this, the image elements are
provided with a dot or line screen. The resolution of the screen is
customarily in the order of magnitude of 60 to 120 L/cm. About 10%
to 30% of the surface remains raised and forms individual screen
dots or line/lattice structures, on which the doctor blade is able
to slide. The diameter of the raised screen dots, depending on
resolution and selected surface coverage, is in that case about 20
.mu.m to 100 .mu.m. Depending on the selected resolution and
surface coverage, the raised dots may also be square or have more
complex angular forms. For printing, the ink which is located in
the depressions between the raised elements is subsequently
transferred to the pad. It is self-evident that on printing, the
raised, fine screen and line elements are subject to massive
mechanical loading. If the doctor blade resistance is inadequate,
individual screen dots will be abraded or fall away entirely, or
scratches or breakouts will be apparent at the line elements.
The depths of engraving are generally between 20 and 30 .mu.m. For
the laser engraving of the pad printing plates of the invention, an
energy input of 10 J/cm.sup.2 up to 20 J/cm.sup.2 is generally
required. At higher energies, the resolution of the fine elements
is adversely affected. The fine elements will no longer be imaged
with accurate detail. They will be damaged, or suffer partial
melting, or burn. If energies are too low, engraving will not be
sufficiently deep.
The lasers may engrave the pad printing plate flatly or, stretched
onto a drum, circularly. The engraving may also take place directly
in the pad printing machine. After having been engraved, the pad
printing cliches are usually cleaned by rinsing with a cleaning
fluid to remove laser dust and other contaminants. For the pad
printing plates of the invention, cleaners based on hydrocarbon
solvents, esters or ketones are readily suitable. Water and
alcohols, conversely, are less suitable, owing to the swelling of
the pad printing cliches.
The doctor blade resistance of the pad printing cliches of the
invention is dependent not only on the composition of the recording
layer but also on the surface quality and the roughness of the
surface. Typical roughness values, measured as averaged roughness
depth Rz according to DIN 4768, ought to be greater than 0.3 .mu.m
and less than 3 .mu.m. At higher roughness values, the plate may
tone, i.e., transfer printing ink to the nonimaged areas. In the
case of entirely smooth layers, surprisingly, the doctor blade
resistance is lower.
The engraved pad printing cliches are subsequently mounted in the
pad printing machine. The cliches are usually fixed magnetically,
something which of course necessitates a magnetic steel support. A
distinction is made between machines with closed and open ink blade
pots. For high-quality prints, machines with a closed blade pot are
preferred. In that case, however, the mechanical load on the
cliches is greater than in the open system.
For printing, solvent-based one-component inks or two-component
inks are employed. It is usual to use printing inks based on
polyester resins. Typical solvents are aromatic or aliphatic
hydrocarbon solvents, cyclohexanone, and acetates. Curing agents
used are usually polyfunctional aliphatic isocyanates. For final
through-curing, these inks often require several days. More
recently, UV-curing printing inks have also increasingly been
used.
A further subject of the present invention, therefore, is a method
for printing a substrate by the pad printing process with a pad
printing cliche of the invention, obtainable by the method
described above, comprising steps (vii) to (x): (vii) fastening the
pad printing cliche in the pad printing machine, (viii) inking the
pad printing cliche with a solvent-based pad printing ink, (ix)
stripping off the excess printing ink by means of a doctor blade,
(x) transferring the printing ink by means of a rubber pad onto the
substrate to be printed.
The invention is elucidated in more detail by the examples
below.
EXAMPLES
Production of Pad Printing Plates
Example 1
A tin-plated steel plate 240 .mu.m thick was coated with a
2-component polyurethane adhesion varnish (2K PU topcoat GM60-6203
from BASF and Desmodur L67MPA/X from Bayer as curing agent in a
ratio of 2:1) in a curtain coater. Following application of the
adhesion varnish, the plate was baked at 250.degree. C. for 1
minute. The average coat weight of the adhesion varnish was 15
.mu.m.
In parallel to this, a PET film of medium roughness (Melinex 383,
layer thickness 125 mm, available from Dupont-Teijin) was coated
with a laser-engravable recording layer. The composition of the
recording layer is reproduced in the table below.
TABLE-US-00001 Fraction solids Component Function Manufacturer (wt
%) Alcotex 72.5 binder Kuraray 63.00 Carbon black IR absorber
Lanxess 27.75 (Pigment Black 7) Syloid ED3 filler Degussa 8.99
Capstone FS-30 flow control Dupont 0.26 assistant Total 100
The components of the recording layer were dissolved in
water/n-propanol in a ratio of 3:1 (solids content 16.3 wt %) and
dispersed in a ball mill for 3 h. The solution was applied by the
metering roller application process on a coating line with double
applicator system. In the first applicator system, a dry film
thickness of 10 .mu.m was applied; in the second applicator system,
a dry film thickness of 20 .mu.m. The web speed was 10 m/min and
the length of the drying tunnel was about 12 m, hence resulting in
a drying time of 72 seconds. Drying was accomplished by heated
circulating air in a countercurrent process. The maximum
temperature of the circulating air in the drier was 145.degree. C.
The PET film coated with the recording layer was subsequently
laminated onto the coated steel plate. n-Propanol was used as a
laminating assistant. The pad printing plates were thereafter
stored at room temperature for 2 days and then processed
further.
Example 2
Procedure as per example 1, but without inorganic filler in the
recording layer.
Example 3
Procedure as per example 1, but the recording layer was
additionally crosslinked with glyoxal.
The composition of the recording layer as per example 3 is
reproduced in the table below.
TABLE-US-00002 Fraction solids Component Function Manufacturer (wt
%) Alcotex 72.5 binder Kuraray 61.35 Carbon black IR absorber
Lanxess 27.02 (Pigment Black 7) Syloid ED3 filler Degussa 8.75
Glyoxal crosslinker BASF 2.62 Capstone FS-30 flow control Dupont
0.26 assistant Total 100
Example 4
Procedure as per example 3, but Silbond 800 EST from Quarzwerke
Group was used as inorganic filler.
Laser Engraving and Printing Tests
The cover film of the pad printing plates from examples 1 to 4 was
removed.
The plates were mounted onto the drum of an IR laser (Thermoflex X
48, Xeikon) and lasered with a resolution of 5080 dpi. The lasered
motif comprised three different screen wedges, the resolution of
the screen selected being 80 L/cm, 100 L/cm, and 120 L/cm. For each
ruling, the surface coverage was varied from 70% to 90%. Surface
coverage in pad printing means the percentage area removed by
engraving, in comparison to the total area.
The power of the laser was 30 watts. The optimum distinctness of
imaging was achieved at a speed of rotation of 3.5 revolutions per
second. This speed of rotation corresponds to an energy input of 15
J/cm.sup.2.
The engraved cliches were subsequently mounted on a pad printing
machine (from Morlock, closed blade pot). The pad printing ink used
was a solvent-based pad printing ink, Marabu TPY980 (white). The
ink contains hydrocarbons, ketones, and acetates as solvents. The
curing agent added was 10% isocyanate curing agent H1 from Marabu.
The cliches were processed with a frequency in each case of 1000
doctor blade operations per hour, and were subjected after 1 hour
in each case to microscopic examination for damage/erosion, etc. As
soon as initial damage, such as the absence of individual screen
elements, was detectable, the test was terminated and the number of
doctor blade operations was recorded.
The results of the printing tests are reproduced in the table
below.
TABLE-US-00003 2 Example 1 (comparative) 3 4 Removal of the easy
easy easy easy cover film Adhesion of the not not not not recording
layer removable removable removable removable Water solubility
soluble soluble insoluble insoluble of the recording layer Laser
energy 14.0 14.0 14.0 14.0 (J/cm.sup.2) Depth of 28 30 31 30
engraving (.mu.m) Dimensions of 15 .times. 15 .mu.m 15 .times. 15
.mu.m 15 .times. 15 .mu.m 15 .times. 15 .mu.m raised elements at
120 L/cm and 90% surface covered Doctor blade 4000 1000 >40000
>40000 resistance Handling difficult difficult easy easy
The cover film was readily removable from all the cliches. The
adhesion to the varnished steel support was high. The recording
layer could no longer be removed from the support. After coating
and drying, the recording layer remained water-soluble in the tests
without crosslinker, meaning that the layer could be dissolved with
fine dispersion. The crosslinked layers, in contrast, were
insoluble in water. There were no problems with the handling of the
crosslinked recording layers. When the noncrosslinked plate
surfaces from examples 1 and 2 were touched, in contrast, they
exhibited significant fingerprints.
The printing plates were engraved with a laser energy of 14
J/cm.sup.2. The depth of engraving of around 30 .mu.m was achieved
in the case of all the cliches. In all of the cliches, fine
elements could be imaged up to a surface coverage of 90%. The
raised, fine elements were approximately square with an edge length
of 15 .mu.m. No melt burr could be seen on any cliche.
A notable feature was the unexpectedly good doctor blade resistance
of the printing plate from example 1 without chemical crosslinker,
which withstood up to 4000 doctor blade operations without damage.
In contrast, the doctor blade resistance of the printing plate
according to example 2 (without inorganic filler) was significantly
poorer. The printing plates of examples 3 and 4, in which the
recording layer was additionally crosslinked chemically, had
excellent doctor blade resistances. After 40 000 doctor blade
operations, these cliches were still undamaged.
* * * * *